1. Field of the Invention
Bacteria multiply rapidly in food, causing it to spoil and resulting in problems associated both with food storage and food safety. A variety of preservation techniques such as canning, freezing, drying, salting, pickling, etc. have been utilized to retard spoilage and extend shelf life. In addition, boiling will kill any bacteria present in food, but the heat required for boiling and any other process requiring heat often damages the flavor, texture and/or nutritional value of the treated foods. Pasteurization, or controlled heat, has been a particularly beneficial treatment from a food safety standpoint and is now required for many liquid foods such as milk and fruit and vegetable juices. However, it also suffers from the problems associated with heat treatments.
This invention relates to a novel liquid food treatment process which utilizes electromagnetic energy (EME), such as microwave or radio frequency energy, for effectively reducing the presence of microorganisms which cause food spoilage or damage, thus providing enhanced food preservation capability without accompanying thermal damage.
2. Description of the Related Art
Most foods interact with high frequency electromagnetic fields such as microwave energy, and a number of studies have been carried out to investigate the effects of these interactions. Results have been inconsistent, however, and there is some debate over whether there are nonthermal effects associated with microwave processing. Mudgett (1982. Food Tech. pp. 109-115), for example, has reported finding no nonthermal effect. Microbial survival and thermal denaturation of heat labile constituents in microwave processing were governed by the same time-temperature relationships as those in conduction heating. Moreover, Goldblith and Wang (1967. Applied Microbiology. pp. 1371-1375) detected no difference when they exposed suspensions of Escherichia coli (E. coli) and Bacillus subtilis to both conventional heating and microwave heating. In addition, Welt et al. (1994. Applied and Environmental Microbiology. vol. 60, no. 2, pp. 482-488) investigated the effect of sublethal temperature treatment with microwave energy on Clostridium sporogenes in potassium phosphate buffer. They found no additional lethality aside from the rapid heating offered by microwave radiation.
On the other hand, Webb and Dodds (1968. Nature. vol. 218, pp. 374-375; 1969. Nature. vol. 222, pp. 1199-1200) reported that cells of E. coli grown in nutrient broth and exposed to microwaves exhibited slowed down cell division and an inhibited metabolic process early in the life span of the cell. Culkin and Fung (1975. J. Milk Food Technol. vol. 38, no. 1, pp. 8-15) studied the effect of microwaves on E. coli and Salmonella typhimurium in tomato soup, vegetable soup and broth, and their data suggested that "the heat generated during the microwave exposure alone is inadequate to fully account for the nature of the lethal effects of microwaves for microorganisms". In addition, Olsen (July, 1965. Food Engineering. pp. 51-53) observed that the numbers of viable spores of Aspergillus niger, Penicillium sp. and Rhizopus nigricans were greatly reduced by microwaves during breadmaking.
Due in part to this controversy, little concentrated effort has been made to develop procedures which effectively utilize electromagnetic energy for purposes of reducing bacterial load and thus preserving and/or increasing the shelf life of perishable foods.